1. Field of the Invention
The present invention concerns an analog control device, especially to an analog controller for an inverter which is fed by a power generating device with direct current and feeds the utility grid with alternate current. The power generating device, preferably solar panel, generates electricity from renewable energy sources.
2. Description of the Related Art
The output power of power generating device that captures solar or wind energy can be changed due to different environmental conditions. Therefore, maintaining optimized power conversion efficiency with stable in-phase output current of an inverter is an essential design consideration for a controller.
With reference to FIG. 7, a system diagram of a conventional two-stage solar-energy power generating apparatus is disclosed. The two-stage solar-energy power generating apparatus includes a solar panel 90, a maximum power point tracking (hereafter “MPPT”) regulator 91 and an inverter 92. The MPPT regulator 91 is disposed at front stage to receive the output voltage and output current from the solar panel 90 and perform MPPT technique for the electrical output of the solar panel 90. The inverter 92 is disposed at back stage to convert the DC power processed by MPPT regulator 91 to AC power, which is then injected into utility grid. The inverter 92 is controlled by a controller 93, which adaptively adjusts the output power of the inverter 92 according to the output voltage of front stage and injects the energy captured from the front stage into utility grid to achieve unity power factor (hereafter “PF”).
Perturb and observe (hereafter “P&O”) algorithm is a typical MPPT method for solar-energy generating system. With reference to FIG. 8, a characteristic curve of the output power versus the output voltage of a solar panel is disclosed. In the characteristic curve, the operating region can be divided into a positive-slope area and a negative-slope area according to the maximum power point. To approach the maximum power point, the P&O algorithm intends to increase the output voltage of the solar panel when the operating point is at the positive-slope region, whereas the P&O algorithm intends to decrease the output voltage of the solar panel when the operating point is at the negative-slope region.
Because the output power of solar-energy power generating system can be influenced by environmental conditions, such as the change of temperature or solar irradiance, the MPPT regulator may be further designed with smart control techniques, such as fuzzy control and neural network techniques, to predict the optimized operating point for obtaining smooth MPPT. However, these techniques are complicated and difficult in implementation.
Conventional controllers for the inverter can be divided into digital circuit's and analog circuit's implementations. The shortcoming of using digital circuits is that the precision of the controller is limited by the number of bits of the microprocessor and is slower in response. Although the existing analog circuit's implementation achieves fast and precise control, they are implemented large in size, expensive and with complicated algorithm.
Therefore, it is necessary to provide an analog controller for an inverter to overcome the existing problems in the conventional technology.